1. Field of the Invention
The present invention generally concerns thermal mass flow sensors, and more particularly, concerns thermal mass flow sensors made of micro-electro-mechanical systems (MEMS) approach, and methods of manufacturing and operating such mass flow sensors.
2. Description of the Related Art
Heat transfer principle has been widely used for mass flow measurements. Thermal mass flow sensors can be found in many applications in industrial process monitor and control, medical gas flow management, and environmental equipments, to name a few. One of the major concerns on the traditional mass flow sensors is the large power consumption used to heating the fluid flow that leads to an uncertainty in low flow measurement domain and restrains on some applications such as gas trade metrology. The difficulties of making identical sensors also make manufacture cost higher than those for other technology. MEMS technology, on the contrary, allows fabrication of thermal mass flow sensors directly on silicon with excellent reproducibility, low power, and high reliability at low cost. MEMS-based thermal mass flow sensors have been received increasing attentions in flow measurement applications.
Thermal mass flow sensors can be classified into three basic categories: anemometers, calorimetric flow sensors, and time-of-flight sensors. For simplicity, these three types of thermal mass flow sensors are hereinafter abbreviated as A-, C-, and T-type mass flow sensors, respectively. Traditional T-type mass flow sensors, such as that disclosed in U.S. Pat. Nos. 5,339,695 and 5,347,876 (Kang), utilize hotwire set apart at a certain distance to measure the heat pulse flight time between two wires so that the fluid flow speed can be determined. Hariadi et al (I. Hariadi, H.-K. Trieu, W. Mokwa, H. Vogt, “Integrated Flow Sensor with Monocrystalline Silicon Membrane Operating in Thermal Time-of-Flight Mode,” The 16th European Conference on Solid-State Transducers, Sep. 15-18, 2002, Prague, Czech Republic) disclose a time-of-flight flow sensor fabricated on Silicon-On-Insulator (SOI) wafers, in which heat pulse is fed to the fluid by a heater and a temperature sensor located downstream detects its delay. Measuring a flight time, the sensors give the velocity of the streaming fluid. However, the pulse will be deformed by the flow velocity profile and broaden at the same time by heat diffusion when it propagates down the stream. This means that the pulse tends to be too broad to be useful for slow flows and thus become inaccurate. Similar approaches have been adapted by U.S. Pat. Nos. 5,533,412 (Jerman) and 6,234,016 (Bonne).
Calorimetric flow sensors usually consist of a heater surrounded by temperature sensitive elements arranged symmetrically downstream and upstream. A moving fluid will carry away heat in the direction of flow and accordingly change the temperature distribution around the heater. The temperature difference between upstream and downstream is measured by the temperature sensitive elements. The output signal is commonly fetched using a Wheatstone bridge circuit, in which a pair of downstream and upstream sensing elements comprises two of its four branches. The output signal, which is a measure of temperature difference, is proportional to the flow velocity initially until a high flow velocity is reached where the temperature difference saturates and then decreases at higher flow velocity. In general, calorimetric flow sensors can accurately measure flows with extremely low velocities. However, calorimetric flow sensors may saturate at high flow velocities and hence have a difficulty to measure flows above a certain level of flow velocity. Many traditional thermal mass flow meters using capillary approach utilize this principle. Sensors made with this principle are disclosed such as in U.S. Pat. Nos. 5,014,552 (Kamiunten) and 6,550,324 (Mayer).
Jiang et al (F. Jiang, Y. C. Tai, C. M. Ho, and W. J. Li, “A Micromachined Polysilicon Hot-Wire Anemometer,” Digest Solid-State Sensors & Actuator Workshop, Hilton Head, S.C., pp. 264-267, 1994) disclose a micro-machined A-type flow sensor comprising of a single element, which is heated and the heat loss of which is measured. This heat loss is dependent on the flow rate of the fluid. This heat loss increases with the flow velocity, and the signal of an anemometer is proportional to the square root of the flow velocity. In general, A-type mass flow sensors are less sensitive and extremely noisy in small flows and hence cannot measure small flows accurately. Nevertheless, A-type mass flow sensors have demonstrated that they are capable of accurately measuring flows with high velocities. Hinkle disclosed in U.S. Pat. No. 5,461,913 that in a capillary tube configuration, a pair of A-type sensor can be installed for improved performance, but yet this capillary by-pass configuration shall not apply to flow in a large conduit.
In summary, a major concern is how to extend the measurable flow rate range to the low flow rate and at the same time to the high flow rate within a single MEMS mass flow sensor. Specifically, for those of ordinary skill in the art there is still a need to provide a MEMS mass flow sensor to expand the measurable flow rate range to low flow velocities with sufficient accuracy and at the same time to keep its capability of accurately measuring flows with high velocities.